In a general manner the burners are conceived and designed so that they may operate irrespective of the feed gas: town or city gas, natural gas or liquefied petroleum gas.
The flames which appear at the outlet holes for the gas/air mixture should remain stable, even in case of variation of the characteristic features of the air/gas mixture.
Several solutions have already been proposed for providing multiple gas burners.
There may at first be cited the burners with a so-called "stabilization by a pilot flame". These burners comprise a cap or cover, the edge or flange of which is perforated with holes, and which is resting upon a support.
The main flames which appear at these holes are stabilized by a small flame generated at the base of and possibly above the main flames. This pilot flame permits re-heating of the air/gas mixture of the main flame and permanently ignites the main flame again.
This solution has now a tendency to be abandoned because the manufacturing cost of the burner is high.
Furthermore this burner may not operate with low gas powers. In this case indeed, the pilot flame has a tendency to extinguish or to become quenched and the main flames then become sensitive to drafts or like air currents. Finally, at a high power of the pilot flame, the burner produces CO.
Other so-called "self-stabilized" burners provide stabilization of the flames owing to a particular shape of the outlet section for the air/gas mixture.
An example of a burner of this kind comprises a cap or cover having a peripheral flange or edge with which the cap rests upon a support. Slits are formed in this flange or edge for the exit of the air/gas mixture.
These slits are narrow and their walls have a trapezoidal profile. Thus to the portion of each slit exhibiting a great thickness corresponds a low egress speed of the mixture, which permits a retaining of the flames, whereas the portion of the slit having a small thickness corresponds a substantial mixture flow rate.
This solution has known a great success but it has gradually been abandoned since it is not adapted to the new techniques using aluminum injection molding.
It is indeed the cap which comprises the outlet holes for the air/gas mixture and it should moreover be made from a material withstanding high temperatures, which is not the case of aluminum.
Another example of a burner of this type is provided in two parts, namely an enamelled cast iron or steel cap which is laid upon a head made from injection molded aluminum.
The head comprises a base which supports a peripheral skirt. It is in this skirt where the outlet apertures for the air/gas mixture are formed.
These apertures are constituted by shallower and deeper slits arranged in alternating relationship. The cap rests directly upon the skirt of the burner head.
The shallow slits throttle the air/gas mixture and produce flames which are shorter than the main flames exiting from the deeper slits. These main flames thus are stabilized.
It could be found that with this kind of burner, the gas did not ignite directly at the contact with the burner head. Since these burners are sensitive to the blow-off of the flames, the air proportion in the air/gas mixture should be limited. Owing to this smaller aeration with primary air, these burners require good secondary aeration.
For that purpose the main slits should be spaced widely and the diameter of the burner head therefore is relatively large. Furthermore it is necessary to provide a relatively large height underneath the grate for promoting the secondary aeration.